What about their built-in cryptocurrency? It's a perfect use for it. They could require payment post-install yet before message can be sent.
True, if they're not even going to allow that for payment then they might as well remove it from the app altogether. Because what's the point if they don't even believe in it themselves.
I've literally never seen anybody mention it, much less use it since it was announced.
Nobody uses that and I think it was pre-mined. They should have implemented Monero but the UX isn't there. Maybe a Monero light wallet server run by Signal.
They probably avoided Monero to not attract the additional scrutiny. They don't even accept donations in Monero.